The present invention relates to an image forming apparatus and fog control method based on electrophotographic technology.
In an image forming apparatus based on electrophotographic technology, a developer bearing member for bearing the developer (toner in the case of a one-component development, and toner and carrier in the case of a two-component development) is moved relative to the photoreceptor with an electrostatic latent image formed thereon, whereby the electrostatic latent image on the photoreceptor is developed. In this case, to ensure that the background fog (toner adhered to the background where toner should not adhere) does not occur, a potential difference is provided between the surface potential of the photoreceptor background portion and the bias potential of the developer bearing member (hereinafter referred to simply as “development bias” in some cases).
However, even if a proper potential difference is provided between the surface potential of the photoreceptor background portion and the bias potential of the developer bearing member, the characteristics of the developer such as the amount of charged toner and quantity of the developer are changed by a change with the passage of time due to large number of printing, environmental condition change and long period of time to be left, with the result that a background fog (hereinafter referred to simply as “fog” in some cases) occurs.
One of the efforts to solve this problem is disclosed in the Patent Document 1 (Japanese Patent Application Publication No. 05-224512) wherein toner density of toner fog is detected by a toner sensor while the development bias is changed before image formation, and the characteristic curve of toner density with respect to development bias is obtained. If the development bias capable of outputting the toner density when toner is no adhered is higher than a reference level, copying operation is carried out by increasing development bias by a predetermined amount, thereby solving the problem caused by a rise in fogging level.
Further, the Patent Document 2 (Japanese Patent Application Publication No. 2003-270875) discloses the technique wherein the amount of fog is detected by a fogging toner detecting sensor before and during image formation, and toner fogging preventive operation (preliminary agitation of developer) is performed either during the process of idling before and after image formation, or at the time of turning on the power again subsequent to turning it off.
However, the time to perform the toner fogging preventive operation described in the aforementioned Japanese Patent Application Publication No. 05-224512 and Japanese Patent Application Publication No. 2003-270875 is either during the process of idling before and after image formation, or at the time of turning on the power again subsequent to turning it off. Toner fogging preventive operation is not carried out in the middle of the job. Thus, a variation occurs to the fogging level in a job requiring a long working time. Despite fog deterioration, image formation continues under the same image formation conditions, with the result that the problem of fog deterioration cannot be solved in the prior art.